Powder coatings are increasingly becoming an important segment of the coatings industry. They offer a wide range of advantages over conventional solvent borne coatings, such as the absence of volatile organic compounds, 95-98% recovery of over-sprayed powder, superior corrosion and chemical resistance, and the like. One major disadvantage, however, particularly for polyester/epoxy hybrid decorative coatings, is their lack of flow-out during cure and the resulting uneven or "orange-peel" appearance. This poor appearance is caused predominantly by the high melt viscosity of the solid carboxy functional polyester resin and the unsatisfactorily high melt viscosity of the solid epoxy resin used to cure the polyester resin. There are several solid epoxy resins based upon bisphenol A which have low melt viscosities, such as D.E.R..TM.661 and D.E.R..TM.662 solid epoxy resins available from The Dow Chemical Company and EPON.TM.1001 and EPON.TM.1002 solid epoxy resins available from Shell Chemical Company. Solid epoxy resins are typically available for powder coatings as a finely divided solid flake, chip or granule. These resins can conceivably be used for powder coatings to provide smooth finishes. However, because of their low softening points, they are found to not be stable upon prolonged periods of storage at ambient temperatures. If stored for long periods of time, the resin flakes, chips or granules tend to clump together (sinter) rendering them unusable until mechanically broken apart. Because of the inherent storage problem associated with low melt viscosity epoxy resins, it would be desirable to have available epoxy resins which not only have very low melt viscosities, but also have adequate physical storage stability as evidenced by their high softening points. These resins would advantageously provide high flow-out and consequently impart very smooth finishes without the need for special resin storage facilities. The coatings industry considers standard epoxy resins which have Mettler softening points of about 90.degree. C. and above to be storage stable whereas those have Mettler softening points below about 85.degree. C. require refrigerated storage. Those standard resins having Mettler softening points between 85.degree. C. and 90.degree. C. may or may not require special storage facilities.
It would be desirable to have available solid epoxy resins which have physical storage stability (a Mettler softening point of at least 85.degree. C.) while possessing a relatively low melt viscosity.